The invention relates to a calender for treating a product web, in particular a paper web, for example a smoothing calender.
DE-U-295 04 034 discloses a calender for treating a product web, which has a plurality of rolls, of which in each case a hard roll and a resilient roll define a nip, the rolls are arranged in an upright with their respective axes displaceable in a loading plane and the end rolls are deflection controlled rolls. An intermediate roll in the roll stack is driven, in order to move the product web through the calender.
In this conventional concept with a central drive, the drive forces are transmitted by friction with the other rolls and by the product web. As a result, deflections at right angles to the loading plane are caused, as shown in FIG. 1 by way of example for a vertical roll stack. The vertical roll stack defines a vertical loading plane 1, in which the rolls 2 to 6 are displaceable, in order to close the roll stack and to set defined linear loads in the nips defined in each case between two rolls.
The central drive of an intermediate roll 3 transmits a drive torque to the respectively adjacent rolls 2 and 4, which are not equipped with their own drive and for their part transmit a drive torque to the adjacent roll 5 not equipped with a drive, and so on. The transmission of the drive torques leads to deflection forces at right angles to the loading plane, which, according to FIG. 1, act in the horizontal direction. The deflection forces are indicated by way of example for the rolls 4 and 5 by the arrows 7, 8 and lead to the erroneous positions of the rolls 4, 5, said positions consisting of displacements at right angles to the loading plane 1. The correct positions are indicated dashed. These erroneous positions of the 2 rolls result in nonuniform web profiles, overloading of the edges of the rolls and a loss of web strength.
The processes in the nip are explained in detail in the publication Pav/Svenka, Der Kompaktkalanderxe2x80x94die Antwort auf die Herausforderung nach hohen Geschwindigkeiten bei der Glandttung und Satinagexe2x80x9d (The compact calenderxe2x80x94the answer to the challenge of high speeds in smoothing and calendering], DAS PAPIER 1985, pp. V178 ff.
In order to avoid erroneous positions of the rolls at right angles to the loading plane, use has been made of rolls with relatively large roll diameters, but without being able to eliminate the disadvantageous erroneous positions completely. In addition, the construction and operation of the calender became complicated as a result of the relatively large roll diameters. Finally, it is disadvantageous that even if the displacement of the rolls at right angles to the loading plane is kept within limits, shear stresses still act on the product web in the nip and, in the case of a product web of paper, can loosen the bonding between the fibers in the web running direction and thereby reduce the strength of the paper.
The object of the invention is therefore to provide a calender which permits non-harmful treatment of the web strength and, at the same time, is cost effective in construction and operation.
This object is achieved by the features of claim 1 or claim 3.
This provides a calender in which the shear stresses acting on the product web are minimized. The rolls can be designed with diameters which are determined by criteria other than their resistance to deformation, for example the critical inherent frequencies.
In particular, rolls with smaller diameters can be used, as a result of which the compressive stress at a given linear load can be increased. Rolls of a smaller diameter also have a lower weight, so that the static (gravitation-induced) forces are low in relative terms and smaller bearings can also be used.
The drives apply the specific power tor the respectively driven roll, this power being composed of re-forming, transporting and loss power. In this case, a distribution of 50:50 to the two nip-forming rolls would be only a rough guide, since, for example, a deflection controlled roll has considerably higher friction losses than a normal solid roll.
The forces which are to be controlled out according to the invention can be measured, for example, in the roll bearings; bearings with force measuring systems incorporated are commercially available. However, it is at least also conceivable to use measurement methods to detect the deflections at right angles to the loading plane that are brought about by such forces.
The invention will be explained in more detail below using the exemplary embodiments illustrated in the appended drawings.